1. Field of the Invention
The present invention relates to surface acoustic wave resonators and particularly to a surface acoustic wave resonator which is small in size, which is low in losses, and which has a high Q and a reduced spurious response.
2. Background Art
Generally, a surface acoustic wave resonator is arranged such that, as shown in FIG. 1, interdigital transducers (hereafter abbreviated as IDTs) 2 and 3, made of an electrically conductive material such as aluminum (A1), and reflectors 4 and 5 are formed on a crystalline substrate 1. The interdigital nature of the transducers is better shown in FIG. 4. An input electrical signal is converted by the input IDT 2 into a surface acoustic wave. The surface acoustic wave oscillates between the reflectors 4 and 5 to provide a resonance which in turn is coupled to an external circuit through the output IDT 3. Such a so-called cavity resonator as described above is widely used.
The IDTs 2 and 3 and the reflectors 4 and 5 are generally made of the same electrically conductive material, resulting in an advantage that only one patterning step will suffice. It is known there is such a relationship of frequency characteristics as shown in FIG. 2 between the reflection coefficient .vertline..GAMMA..vertline. and the radiation conductance Ga of the IDT.
On the other hand, there have been a number of reports as to resonators having a reflector structure of the group type in which IDTs are made of ordinary electrically conductive material and reflectors are formed with grooves on their crystalline substrates by a dry etching technique.
In the former case, the peak frequency f.sub.R of the reflection coefficient .vertline..GAMMA..vertline. and the peak frequency f.sub.T of the reflection conductance Ga are not coincident with each other but they are related to each other by the following inequality: EQU f.sub.T &gt;f.sub.R
FIG. 3 shows a typical example of this characteristic.
In this case, there have been such disadvantages that a maximum resonance frequency f.sub.o exists on the low frequency side of a stop band (SB) of a reflector, the sharpness of the resonance Q is degraded (the Q sharpness varies with its maximum value), the level difference between the resonance peak and the second peak, that is, a so-called suprious response (hereafter abbreviated to SR), is also small, and so on. Here SR is a positive value so that a high SR corresponds to a reduced spurious response. These disadvantages are caused, as seen from FIG. 2, by the fact that the radiation conductance Ga of the IDT is lower at the frequency f.sub.R than the maximum value thereof and the characteristics of the IDT are not sufficiently adequately used. In order to obtain a resonator having a high Q value with the same structure, it is necessary to increase the number of gratings of the reflector, resulting in a limit in miniaturization.
In the grooved structure, on the other hand, there is an advantage that the reflection coefficient .vertline..GAMMA..vertline. for each reflector increases and resonance characteristics equivalent to those of the ungrooved structure can be obtained with a smaller number of reflectors than that in the ungrooved case.
There is a disadvantage in the grooved structure, however, in that in order to make the reflector have a grooved structure, the patterning process becomes complicated, resulting in poor mass production.
These and other objects, advantages, and features of the present invention will become more apparent as the description proceeds, when considered with the accompanying drawings.